1. Field of the Invention
The present invention generally relates to a power circuit, and especially to a power circuit supplying power to a load.
2. Description of the Related Art
In recent years, there has been a demand for devices such as a cell phone, a portable music player, and a digital camera utilizing digital data to cooperate with a personal computer. Accordingly, the devices such as a cell phone, a portable music player, and a digital camera include a USB port having high general-purpose properties. Therefore, the devices such as a cell phone, a portable music player, and a digital camera can be directly connected to the personal computer.
The devices such as a cell phone, a portable music player, and a digital camera are normally driven by batteries. In a case where these devices are used while they are connected to the personal computer, when transmitting data, the batteries may die out (become discharged). Thus, a system may shut down, and there is a risk of losing the data being transmitted. Accordingly, when using these devices while they are connected to a personal computer, the power is supplied to the devices from a commercial power source. After an alternating current supplied from the commercial power source is converted to direct current by an AC adapter, the direct current is supplied to the batteries and the systems.
It is indispensable to connect the AC adapter to these kinds of devices. Thus, they are inconvenient in operability and portability. Accordingly, a system is desired which can supply power to the batteries and systems without the AC adapter.
As for a system enabling power supply to the batteries and systems without the AC adapter, Japanese Laid-Open Patent Application Publication No. 2002-300256 discloses a method of obtaining power from a power line such as an interface, e.g., USB, and supplying the power to the systems.
Accordingly, the interface such as USB includes plural abilities of supplying power. For example the interface includes a spec (specification) that the supply of electric current is 100 mA or less. On the other hand, in another spec the supply of electric current is at least 500 mA.
Therefore, when supplying power to the batteries and systems from the interface including plural specs of abilities of supplying power, it is necessary to supply the power corresponding to the specs of the interface.